1. Technical Field
The disclosure generally relates to light scattering, detection, and characterizing particle systems.
2. Description of the Related Art
Light scattering can provide a non-destructive means of obtaining information about an object from a distance. As use herein, the term “light” refers to electromagnetic radiation over the entire electromagnetic spectrum including, but not limited to, visible light, ultraviolet light, infrared light, and near infrared light. The object can be a surface, like that of the Earth observed from space or of a computer wafer in a clean-room environment. The object can also be a cloud consisting of smoke, ice crystals, pollutants, biological agents, or any number of other things about which information is desired. In many scattering configurations, the light source and detector are in close proximity. For instance, in monitoring the atmosphere for a cloud of particles using Light Detection and Ranging (LIDAR), a laser and a detector located on the same apparatus are used. This is primarily for convenience, as it requires only one physical station to set up for instrumentation. In addition, for observations from a distance it makes alignment much simpler. When the source and the detector are each separate instruments, they cannot simultaneously detect and receive light at the same location as there is always some physical separation between them; therefore, the light that scatters from the object of interest cannot be measured by the detector in the exact backscatter direction, which is located 180 degrees from forward scatter. It is measured at some finite angle β, the backscatter angle, measured from the exact backscatter direction. This finite angular extent can have implications in the interpretation of the resulting data.
Various physical phenomena occur when light traveling from a source to an object is scattered back in the direction of the source, i.e., in the backscatter region. In sensing terrain, for instance, no shadows are seen in the exact backscatter direction, but as β increases, shadows will become visible that will reduce the signal on the detector. This is sometimes referred to as the shadowing effect. This shadowing contains information on the morphology and polydispersity of the particles in the sample. In addition, multiple scattering of rays by points on the object causes the rays to interfere with each other. In the backscatter region, rays that travel reciprocal paths interfere constructively, resulting in an increase in signal intensity on the detector. This is sometimes referred to as the coherent backscattering effect or backscattering surge. Accompanying the coherent backscattering effect is the polarization opposition effect—light has zero polarization in the exact backscatter region, but has a negative polarization state at angles slightly off the exact backscatter direction The amount of signal intensity increase in the backscatter region and the rate of fall-off of the polarization are determined by the morphological and chemical properties of the object; hence, a measurement of these light-scattering properties contains important information about the object.
The properties of the absolute intensity and rate of change of intensity and polarization state in the backscatter region are of interest for characterizing objects. Most remote-measuring techniques like LIDAR do not measure light in the exact backscatter direction (β=0), so it is difficult to interpret the information that is retrieved. In addition, it is desirable to know the scattering properties as a function of backscatter angle β, i.e. to have measurements at multiple angles across the backscatter region. One way of decreasing the angle β is to make the distance to the object extremely large or to reduce the distance between the source and the detector; however, β still remains finite.